Recently a method is proposed to record and play back a recording bit smaller than the size of a light beam on a recording medium (hereinafter referred to as a spot diameter) in order to improve recording density of the recording medium such as a magneto-optical recording disk.
Usually intensity of the light beam has Gaussian distribution since the light beam is converged to a diffraction limit by a condenser lens in optical recording, so the temperature of the recording medium also has Gaussian distribution. Therefore, a portion having higher temperature than a predetermined temperature is smaller than the spot diameter. Then, if only the portion having higher temperature than the predetermined temperature can be concerned with recording and playback, the recording density will be improved remarkably.
However, the recording density in a conventional magneto-optical recording medium depends on the spot diameter used for recording and playback. That is, in the conventional magneto-optical recording medium, if a recording bit is smaller than a spot diameter of a light beam, there are plural recording bits in the spot. Therefore, signals from the recording bits cannot be separated. Then, if the recording bit is smaller than the spot diameter, interference by unwanted signals due to neighboring recording bits, which is the cause of generating noises, called crosstalk, grows large at playback. That limits improvement of the recording density due to making the recording bits smaller.